Anti-tumor cytotoxic cell activity generated in vitro by mouse spleen cells against syngeneic MCA-induced tumors is mediated by relatively large lymphocytes lacking detectable surface immunoglobulin and partially sensitive to anti-theta and complement. Cytotoxic cells fail to ingest iron particles. The anti-tumor cytotoxic activity can be generated in a fully syngeneic system and thus is not due to foreign serum components. Tumor associated immunosuppression of in vitro cytotoxic cellular immune responses is markedly reduced by using a fully syngeneic environment.